Slabs of polyurethane foam may be formed in a method that includes the steps of introducing the liquid foam-forming mixture into the bottom portion of an open trough, and allowing the foam mixture to expand in the trough until it spills over a weir or pour plate at the top of the trough onto a moving channel-shaped conveyor. As the foam-forming mixture fills the trough, it begins to change from a liquid to a froth. This froth, the partially expanded foam, then spills over the weir and onto the paper or plastic-lined conveyor that moves away from the trough at a steady rate. The foam completes the transition from liquid to solid foam as it travels along the conveyor. The cured foam slab may then be cut to any desired shape. Such prior apparatus for manufacturing foam slabs are disclosed in U.S. Pat. Nos. 4,074,960, 3,832,099 and 3,786,122.
Various configurations for the trough have been proposed, including means to adjust to length of the trough, means to vary the number of inlet ports at the bottom of the trough, and the choice of materials used to make the trough. The interior surfaces of all of these prior trough configurations will in time become coated with reacting foam as a production run progresses. As the inner surfaces of the trough get coated with more and more of the foam, the volume of the trough in which the expanding foam mixture can rise is correspondingly reduced. Under such conditions, the foaming mass that exits the top of the trough is less viscous than anticipated, and tends to run too quickly down the inclined pour plate onto the conveyor. The resultant foam block may have serious defects when this occurs. In addition pieces of the built-up cured foam can break off the inside surfaces of the trough and become imbedded in the still-reacting foam mixture. These pieces cause additional serious defects in the resultant foam block. Ultimately, the trough surfaces become so built-up with cured foam that production must be halted to remove this build up.